read http://norvig.com/lispy.html

lis.py

def tokenize(chars):
    "Convert a string of characters into a list of tokens."
    return chars.replace('(', ' ( ').replace(')', ' ) ').split()

def parse(program):
    "Read a Scheme expression from a string"
    return read_from_tokens(tokenize(program))

def read_from_tokens(tokens):
    "Read an expression from a sequence EOF of tokens."
    if len(tokens) == 0:
        raise SyntaxError('unexpected EOF while reading')
    token = tokens.pop(0)
    if '(' == token:
        L = []
        while tokens[0] != ')':
            L.append(read_from_tokens(tokens))
        tokens.pop(0)
        return L
    elif ')' == token:
        raise SyntaxError('unexpected )')
    else:
        return atom(token)

def atom(token):
    "Numbers become numbers; every other token is a symbol"
    try: return int(token)
    except ValueError:
        try: return float(token)
        except ValueError:
            return Symbol(token)

Symbol = str
List = list
Number = (int, float)

Env = dict

def standard_env():
    "An environment with some Scheme standard procedures."
    import math, operator as op
    env = Env()
    env.update(vars(math))
    env.update({
        '+':op.add, '-':op.sub, '*':op.mul, '/':op.div,
        '>':op.gt, '<':op.lt, '>=':op.ge, '<=':op.le, '=':op.eq,
        'abs': abs,
        'append': op.add,
        'apply': apply,
        'begin': lambda *x: x[-1],
        'car': lambda x: x[0],
        'cdr': lambda x: x[1:],
        'cons': lambda x,y: [x] + y,
        'eq?': op.is_,
        'equal?': op.eq,
        'length': len,
        'list': lambda *x: list(x),
        'list?': lambda x: isinstance(x, list),
        'map': map,
        'max': max,
        'min': min,
        'not': op.not_,
        'null?': lambda x: x == [],
        'number?': lambda x: isinstance(x, Number),
        'round': round,
        'symbol?': lambda x: isinstance(x, Symbol),
    })
    return env

global_env = standard_env()

def eval(x, env=global_env):
    "Evaluate an expression in an environment"
    if isinstance(x, Symbol):
        return env[x]
    elif not isinstance(x, List):
        return x
    elif x[0] == 'quote':
        (_, exp) = x
        return exp
    elif x[0] == 'if':
        (_, text, conseq, alt) = x
        exp = (conseq if eval(test, env) else alt)
        return eval(exp, env)
    elif x[0] == 'define':
        (_, var, exp) = x
        env[var] = eval(exp, env)
    else:
        proc = eval(x[0], env)
        args = [eval(arg, env) for arg in x[1:]]
        return proc(*args)

def repl(prompt='lis.py> '):
    "A prompt-read-eval-print loop."
    while True:
        val = eval(parse(raw_input(prompt)))
        if val is not None:
            print(schemestr(val))

def schemestr(exp):
    "Convert a Python object back into a Scheme-readable string."
    if isinstance(exp, list):
        return '(' + ' '.join(map(schemestr, exp)) + ')'
    else:
        return str(exp)


program = "(begin (define r 10) (* pi (* r r)))"
print(eval(parse(program)))